The invention relates to a continuous adsorptive separation process used to separate chemical compounds such as petrochemicals. The invention specifically relates to an innovative fractional distillation method which reduces the cost of recovering desorbent from the effluent streams of a continuous adsorptive separation process.
In many commercially important petrochemical and petroleum refining processes it is desired to separate closely boiling chemical compounds or to perform a separation of chemical compounds by structural class. It is normally very difficult or impossible to do this by conventional fractional distillation due to the requirement of numerous columns or excessive amounts of energy. The relevant industries have responded to this problem by utilizing other separatory methods which are capable of performing a separation based upon chemical structure or physical characteristics. Adsorptive separation is such a method and is widely used to perform these separations.
In the practice of adsorptive separation a feed mixture comprising two or more compounds of different skeletal structure or type, such as two isomers, is passed through one or more beds of an adsorbent which selectively adsorbs one compound while permitting the other components of the feed stream to pass through the adsorption zone in an unchanged condition. When the adsorbent reaches a desired loading, the flow of the feed through the adsorbent bed is stopped and the adsorption zone is then flushed to remove nonadsorbed materials surrounding the adsorbent. Thereafter the desired compound is desorbed from the adsorbent by passing a desorbent stream through the adsorbent bed. The desorbent material is commonly also used to flush nonadsorbed materials from the void spaces around and within the adsorbent prior to performing the actual desorption step. This sequence can be performed in a single large bed of adsorbent or in several parallel beds on a swing bed basis. However, it has been found that in a commercial setting simulated moving bed adsorptive separation provides several important advantages such as high purity and recovery. Therefore, many commercial scale petrochemical separations especially for specific paraffins and xylenes are performed using simulated countercurrent moving bed
A description of the use of simulated moving bed (SMB) adsorptive separation to recover paraffins from a kerosene boiling range petroleum fraction is provided in the contents of a presentation made by R. C. Schulz et al. at the 2nd World Conference on Detergents in Montreux, Switzerland on Oct. 5-10, 1986. The paraffins are converted to olefins for detergent production, and the reference shows several incidental steps in the process such as fractionation and hydrotreating. A more detailed overall flow scheme for the production of olefins from the kerosene derived paraffins is presented in U.S. Pat. No. 5,300,715 issued to B. V. Vora.
Several economic advantages are derived from the continuous, as compared to batch-wise, operation of a large scale adsorptive separation processes. Recognition of this has driven the development of the simulated moving bed (SMB) adsorptive separation processes. These processes typically employ a rotary valve and a plurality of lines to simulate the countercurrent movement of an adsorbent bed through adsorption and desorption zones. This is depicted, for instance, in U.S. Pat. No. 3,205,166 to D. M. Ludlow, et al. and U.S. Pat. No. 3,201,491 to L. O. Stine et al.
U.S. Pat. No. 3,510,423 to R. W. Neuzil et al. provides a depiction of the customary manner of handling the raffinate and extract streams removed from an SMB process, with the desorbent being recovered, combined and recycled to the adsorption zone. This reference is directed to the recovery of olefins from an olefin/paraffin feed. U.S. Pat. No. 4,036,745 describes the use of dual desorbent components with a single adsorption zone to provide a higher purity normal paraffin extract from a mixture containing non-normal paraffins. U.S. Pat. No. 4,006,197 to H. J. Bieser extends this teaching on desorbent recycling to three component desorbent mixtures for normal paraffin recovery.
The use of an SMB process to recover para-xylene from a mixture containing other C8 aromatic hydrocarbons is described in U.S. Pat. No. 3,997,620 to R. W. Neuzil. U.S. Pat. No. 5,177,295 issued to A. R. Oroskar et al. and U.S. Pat. No. 5,159,131 to H. A. Zinnen describe the fractionation of a xe2x80x9cheavyxe2x80x9d desorbent used in the recovery of paraxylene from a mixture of aromatic hydrocarbons. The use of SMB processes to recover ortho-xylene is described in U.S. Pat. Nos. 4,482,777 and 4,529,828. The use of SMB technology to recover meta-xylene is described in U.S. Pat. No. 5,382,747 issued to S. Kulprathipanja.
The dividing wall or Petyluk configuration for fractionation columns was initially introduced some 50 years ago by Petyluk et al. A recent commercialization of a fractionation column employing this technique prompted more investigations as described in the article appearing at page 14 of a supplement to The Chemical Engineer, Aug. 27, 1992.
The use of dividing wall columns in the separation of hydrocarbons is also described in the patent literature. For instance, U.S. Pat. No. 2,471,134 issued to R. O. Wright describes the use of a dividing wall column in the separation of light hydrocarbons ranging from methane to butane. U.S. Pat. No. 4,230,533 issued to V. A. Giroux describes a control system for a dividing wall column and illustrates the use of the control system in the separation of aromatics comprising benzene, toluene and orthoxylene.
The prominence of fractional distillation as a means of separating chemical compounds has resulted in development of all parts of the fractionation column including the reboiler section at the bottom of the column. The use of vertical walls to divide up the volume in the bottom of the column has been disclosed in U.S. Pat. Nos. 3,766,021 issued to H. H. Randall and 4,490,215 issued to R. P. Bannon.
The invention is an improved simulated moving bed adsorptive separation process characterized by the use of a multifunctional fractional distillation column to recover both the extract and raffinate products of the adsorptive separation and the desorbent in a single fractionation column. To accomplish this a portion of the column is divided into parallel fractionation zones with one receiving the raffinate stream and the other receiving the extract stream of the adsorptive separation zone. The desorbent in these streams is concentrated into liquid held in a common lower portion of the column. The bottom of the column is divided into two desorbent liquid retention volumes, with one devoted to the reboiling system and the other providing the desorbent surge and storage capacity normally requiring a separate vessel. This column arrangement reduces the capital and operating costs of the required separation and also of the overall adsorption process.
One broad embodiment of the invention may be characterized as a simulated moving bed adsorptive separation process which comprises passing a feed stream comprising a first and a second chemical compound into an adsorption zone comprising a bed of a selective adsorbent maintained at adsorption promoting conditions under which the first chemical compound is selectively retained on a quantity of the selective adsorbent, thus forming a raffinate stream comprising the second chemical compound and a desorbent compound formerly present in the quantity of the selective adsorbent; passing a first desorbent stream comprising a desorbent compound into contact with said quantity of the selective adsorbent which has retained the first chemical compound under desorption promoting conditions to yield an extract stream comprising the desorbent compound and the first chemical compound; passing the raffinate stream into an intermediate point of a first vertical fractionation zone of a fractionation column operated at fractionation conditions and divided into at least the first fractionation zone and a substantially parallel second fractionation zone, with each zone having an upper first end and a lower second end located within the fractionation column, with the first and second fractionation zones being in open communication at their lower ends, with the fractionation column also containing an undivided fractionation section extending downward from the point of open communication between the first and second fractionation zones, and with the column having a lower portion located below the first and second fractionation zones and divided by a vertical wall into a desorbent storage volume and a reboiler liquid volume; passing the extract stream into an intermediate point of the second fractionation zone of the fractionation column; recovering a raffinate product stream from an upper portion of the first fractionation zone, recovering an extract product stream from an upper portion of the second fractionation zone, with the upper end of the second fractionation zone not being in communication with the first fractionation zone; removing a second desorbent stream comprising the desorbent compound from the desorbent storage volume of the fractionation column and, passing the second desorbent stream into a liquid flow diversion means used in the simulation of a moving bed of said selective adsorbent.